1. Field of the Invention
The present invention relates to a camera equipped with a viewfinder optical system that guides the luminous flux of the subject to the viewfinder, and a picture taking optical system that guides the luminous flux of the subject to an image pickup element, where a variable aperture is positioned so that it affects light entering the picture taking optical system and does not affect light entering the viewfinder optical system.
2. Description of the Related Art
FIG. 1 is a diagram illustrating a conventional electronic still camera comprising a quick-return mirror 2 arranged behind a picture taking lens 1. Picture taking lens 1 includes a built-in variable aperture 1a. The diameter of variable aperture 1a varies from a maximum aperture value (representing a fully open aperture) to a minimum aperture value in order to control the quantity of light passing through variable aperture 1a. A focusing screen 3, a field lens 4, and a pentagonal prism 5 are arranged above quick-return mirror 2. An eyepiece 6 and a photometric element 7 are positioned to the right of pentagonal prism 5. A shutter 8, a field lens 9 and a mirror 10 are arranged behind quick-return mirror 2. Shutter 8 is a conventional shutter which comprises front blades (not illustrated) and rear blades (not illustrated). A mirror 11 is positioned in the path of the reflected light from mirror 10 and reflects the reflected light in a direction which is parallel to the optical axis of picture taking lens 1. A relay lens 12, an optical low-pass filter 13, and an image pickup element 14 are arranged to receive the light reflected by mirror 11. Luminous flux condensed by picture taking lens 1 passes through variable aperture 1a, is reflected by quick-return mirror 2, and is formed into an image on focusing screen 3. The image formed on focusing screen 3 is observed by a photographer 15 through field lens 4, pentagonal prism 5 and eyepiece 6. Also, a portion of the luminous flux diffused by focusing screen 3 enters photometric element 7.
A release button (not illustrated) is positioned on the camera so that a photographer can press the release button. The release button typically has two positions: (1) a half-push position in which the release button is pushed half-way by the photographer, and (2) a full-push position in which the release button is fully pushed by the photographer. A release switch (not illustrated) turns ON when the release button is pressed to the full-push position. Photometry is performed when the photographer presses the release button to the half-push position. When the photographer presses the release button to the full-push position, quick-return mirror 2 moves out of the optical path and shutter opens 8, thereby taking a photograph. The luminous flux condensed by picture taking lens 1 is formed into an image on a primary image forming plane 16. The image then is reformed on image pickup element 14 through the action of relay lens 12 via field lens 9 and mirrors 10 and 11. Through optical low-pass filter 13, which is positioned in front of image pickup element 14, a specified spatial frequency component is eliminated. Variable aperture 1a is driven by stepping motor (not illustrated). The image formed on image pickup element 14 is converted to a charge signal and stored. The stored charge signal is output as an electrical signal, converted to a digital amount and stored as image data in a storage device (not illustrated) by a control circuit (not illustrated). The storage device is typically an image memory.
In the above-described conventional still camera, setting of the focus and the composition are performed while a photographer 15 is observing the image formed on focusing screen 3 before picture taking. Variable aperture 1a is stopped down by a control aperture value before picture taking is performed. However, when stopping down of variable aperture 1a is performed before picture taking, the image formed on focusing screen 3 becomes darker, thereby making it difficult to obtain good focusing and to determine the composition. Also, focusing screen 3 is not a perfect diffuser. Therefore, when the luminous flux is stopped down to a certain extent by variable aperture 1a, the luminous flux is not diffused by focusing screen 3, no light enters photometric element 7, and photometry becomes impossible. Therefore, in a conventional electronic still camera, a well-known "exposure measurement at open aperture method" is employed in which the variable aperture is opened during photometry.
The following is a description of the "exposure measurement at open aperture method". Photometry is performed with variable aperture 1a in a fully open position by photometric element 7 when the photographer presses the release button to a half-push position. A control aperture value is computed based on the results of a photometric measurement by photometric element 7. Then, when the release switch is turned ON by the release button being pressed to a full-push position, quick-return mirror 2 moves upwards in a mirror-up operation and, simultaneously, a stepping motor (not illustrated) operates to control variable aperture 1a, and variable aperture 1a is stopped down from the open position to a control aperture value which represents a control aperture position. Supply of power to the stepping motor continues during exposure to reliably maintain the control aperture position. When a predetermined time has been reached after the release button is pressed to the full-push position, shutter 8 opens for a specific shutter period and the luminous flux of the subject is reformed into an image on image-pickup element 14. Then, when an operation complete signal of shutter 8 is received, variable aperture 1a returns to the open position simultaneously with a mirror-down operation of mirror 2. This open position of variable aperture 1a is stably maintained by the supplying of power to the stepping motor.
In a conventional electronic still camera using the above-described "exposure measurement at open aperture method", aperture 1a is positioned in the optical path from picture taking lens 1 to photometric element 7 and eyepiece 6 via quick-return mirror 2. Therefore, when an attempt is made to stably maintain aperture 1a at an open value to obtain good focusing, composition setting or accurate photometry, power must be continuously supplied to the stepping motor since, if the supply of power to stepping motor is eliminated, there is a danger that the aperture diameter of aperture 1a will change from vibrations or external forces exerted on the camera. As a result of the need to continuously supply power to the stepping motor, electric power consumption of the stepping motor increases and battery power is wastefully consumed.
FIG. 2 is a timing chart illustrating the operational relationship between the release switch, the front blades of the shutter, and the variable aperture in a case where the control aperture value is relatively near the initial aperture value (fully open aperture value) at the time when the release switch becomes ON. As indicated by FIG. 2, shutter 8 opens aperture 1a when the front blades of shutter 8 passes over aperture 1a. T is the time from the release switch being turned ON (that is, the release button is pressed to the full-push position) to when shutter 8 begins to open. T is a constant. "Hatching" in FIG. 2 represents the time from the release switch being turned ON (aperture control start point at time T100) to when aperture control is completed at time T200. T1 represents the time from aperture control being completed to when shutter 8 begins to open at time T300 and, during time T1, a current for maintaining the control aperture value is supplied to the stepping motor.
FIG. 3 is a timing chart illustrating the operational relationship between the release switch, the front blades of the shutter, and the variable aperture in a case where the control aperture value is relatively far from the initial aperture value at the time of the release switch becomes ON. In FIG. 3, "hatching" represents the time from the release switch being turned ON (aperture control start point at time T100) to when aperture control is completed at time T200. T2 represents the time from aperture control being completed to when shutter 8 begins to open at time T300. During time T2, the current for maintaining the control aperture value is supplied to the stepping motor.
As illustrated in FIGS. 2 and 3, the time T, or "time lag", from the release button being pressed to when the shutter begins to open is constant. This time lag T is based on the time required to change the variable aperture 1a (which is at the fully open aperture value during the photometry operation) to the minimum aperture value. Therefore, in a conventional camera which requires aperture 1a to be set to the fully open aperture value during the photometry operation, a relatively long time lag T is set. Therefore, a high probability exists that a rare photograph opportunity will be missed.
Time lag T is constant. However, as indicated by T1 and T2 in FIGS. 2 and 3, respectively, and regardless of the size of the gap from the initial aperture value at the time of release (the fully open aperture value) to the control aperture value, there is an amount of time proportional to T1 and T2 from the completion of aperture control to the shutter opening operation in which power must be supplied to the stepping motor to stably maintain the control aperture value during this period. Also, power consumption is increased because the current required at this time is larger than when the stepping motor rotated.
FIG. 4 is a flow chart illustrating a processing sequence of a conventional electronic still camera during continuous shooting. The processing sequence begins with the pressing of the release button to a half-push position. In step S21, the control aperture value and the shutter period are computed based on brightness data measured by photometric element 7. In step S22, aperture 1a is driven to the computed control aperture value. When the release button is pressed to the full-push position, the system proceeds to step S23 where the shutter opening operation is implemented in accordance with the shutter period computed in step S21, and picture taking is performed. In step S24, a determination is made as to whether the release button has been pressed to the full-push position. If the release button was pressed to the full-push position, it is determined that continuous shooting is in progress. Therefore, the system moves to step S25 where the variable aperture 1a is moved to the open aperture value and, thereafter, the system returns to step S21. If the release button is not pressed to the full-push position in step S24, the process moves to step S26 where processing ends. As illustrated by FIG. 4, it is necessary to return variable aperture 1a to the fully open aperture value as the photographing of each frame is completed. When variable aperture 1a is stopped down (especially when aperture 1a is stopped down to the minimum aperture value), time is consumed in returning variable aperture 1a to the fully open aperture value. For this reason, time is required before the next photograph can be taken, the number of pictures taken in a unit of time drops, and the continuous shooting speed during continuous picture drops.
FIG. 5 is a timing chart illustrating the operational relationship between the release switch, variable aperture 1a, and shutter 8 during continuous shooting. In FIG. 5, T3 (hereafter referred to as "aperture control time") is the time required to drive variable aperture 1a to the control aperture position, and T4 is the time required to return variable aperture 1a from the control aperture position to the fully open aperture value and represents the aperture open time. As shown in FIGS. 4 and 5, during continuous shooting, variable aperture 1a must be returned to the fully open aperture value each time a frame is photographed. Therefore, continuous shooting at high speed is not possible.
FIG. 6 illustrates a light travel path from picture taking lens 1 to image pickup element 14, with the travel path illustrated as linearly extended for simplification of the illustration. Variable aperture 1a is stopped down in FIG. 6. The luminous flux enters the corner of primary image plane 16 and enters the corner of image pickup element 14. If the angle of the luminous flux which enters field lens 9 is angle .THETA., angle .THETA. must be within a specified range for the luminous flux to enter relay lens 12.
FIG. 7 illustrates a camera with a picture taking lens 201 having a long focal length and an aperture 201a which is stopped down to a certain extent. In this situation, the luminous flux does not enter relay lens 12 because an exit pupil position of picture taking lens 201 is far from field lens 9 and because the angle of incidence .THETA. of the luminous flux which enters the corner of primary image forming plane 16 becomes large. Moreover, FIG. 8 illustrates a camera with a picture taking lens 301 having a short focal length and an aperture 301a which is stopped down to a certain extent. In this situation, the luminous flux does not enter relay lens 12 because an exit pupil position of picture taking lens 301 is near field lens 9 and because the angle of incidence .THETA. of the luminous flux which enters the corner of primary image forming plane 16 becomes small.
As illustrated by FIGS. 7 and 8, whether or not the luminous flux enters relay lens 12 changes according to the focal length and the exit pupil position of the picture taking lens and the aperture value. Specifically, luminous flux from the four corners of primary image forming plane 16 does not enter relay lens 12, but it does enter focusing screen 3 (which is at a conjugate position with primary image forming plane 16). Therefore, the photographer does not know that a portion of the luminous flux does not enter image pickup element 14. As a result, images which are darkened at the four corners of primary image forming plane 16 will be recorded without the photographer being aware of the darkened image.